All the irregular shapes (shapes that have a concave corner) that can be formed with four or less unit cubes are the ones pictured below:

It is a surprising fact, discovered by Piet Hien, that these shapes can be arranged to form a cube with side length 3. Additionally, the cube can be formed in 240 different ways. This puzzle is called the Soma Cube. The easiest way to make your own set is to purchase 27 wooden unit cubes (they are inexpensive and available at most craft stores) and glue them into the configurations pictured above.